The present invention represents an improvement over known "correctable" ribbons and carbons such as disclosed, for instance, in U.S. Pat. Nos. 3,825,437 and 3,825,470. "Correctable" ribbons were developed in order to facilitate the making of clean corrections by means of a correcting typewriter, whereby an erroneous typed image can be removed cleanly from a copy sheet by overstriking the image with an adhesive ribbon or tape. The "correctable" ribbon composition is formulated so as to be dry, resistant to oil migration into the copy paper onto which it is typed, hard and strongly cohesive, so as to be completely and cleanly removable from the copy paper, and yet brittle so as to have good pressure-transfer properties or frangibility.
In many ways, "correctable" transfer compositions represent a step backward in the art because they must be so dry and brittle that they have poor adhesion for their film foundation and tend to be removable if contacted by the fingers or by the ribbon-transport guides of the typewriter. Also, such compositions must have weak adhesion for the copy paper and frequently the type images are incomplete or have poor edge profile or sharpness.
Conventional correctable ribbons having polyolefin film foundations, such as polyethylene and polypropylene, have been found to be unsatisfactory for the production of correctable transfer elements having good shelf life and capable of producing typed images which are sharp and clear and free of image fill-in. Thus, the more expensive and less deformable polyethylene terephthalate films are being used to avoid these problems.
However, we have found that such problems are not due to the nature of the film foundation but rather are due to the fact that the oily plasticizer migrates from the correctable transfer composition and actually penetrates the polyolefin film foundation to the rear surface thereof, where it is picked up and accumulated on the type faces, such as present on a "golf ball" type element, and most particularly within the enclosed centers of characters such as "o," "p," "e," etc. The accumulated oil attracts dust and paper fibers to the type faces and reduces the ability of such type faces to make uniform, sharp contact with the rear surface of the film, which contact is necessary to the transfer of sharp, clear images free of fill-in.
Mineral oil and fatty acid esters, such as butyl stearate, are used in prior known correctable transfer compositions and function to modify the normally-hard resinous binder material by disrupting its continuity and rendering it brittle so that the resinous coating is frangible and pressure-transferable in image form. However, we have discovered that mineral oils, fatty acid esters and hydroxylated fatty acids, such as ricinoleic acid (castor oil) are penetrants for polyolefin film foundations and prevent the use of such beneficial film foundations as supports for complete release or correctable transfer elements.